1. Field of the Invention
The invention relates to DC offset estimation system and method.
2. Description of the Related Art
In wireless communication, DC offset is generated both in transmitter and receiver. At the transmitter, one possible reason for DC offset generation is that the DAC (digital to analog converter) is not ideal, and the input digital data may not even contain a DC value, the converted data might also contain DC offset. Additionally, the nonlinear feature of other analog elements of the transmitter may also cause unwanted DC offset. As to the receiver, if the receiver has Zero IF configuration, the DC offset effect is more obvious. FIG. 1 and FIG. 2 show schematic diagrams of DC offset generation in a receiver. Since the Zero IF receiver utilizes only one stage conversion to convert a radio frequency signal to a baseband signal, the signal COS ωLOt easily leaks to combine with the input signal S1 if the conductive line to the mixer is not completely isolated from the conductive line for transmitting the input signal S1, thus, the next one radio frequency signal carries the unwanted leakage signal, and the converted baseband signal carries a DC offset value. FIG. 2 shows another DC offset generation event. Since the Zero IF receiver converts a radio frequency signal to a baseband signal dircectly, the input signal S1 easily leaks to combine with the signal COS ωLOt if the conductive line to the mixer is not completely isolated from the conductive line for transmitting the input signal S1.
FIG. 3 is a schematic block diagram of a conventional OFDM receiver 300 for performing DC estimation and compensation for long training symbols. This implementation attempts to determine the estimated value of the signal, s(n), in order to estimate the DC offset. The signal s(n) shown in FIG. 3 may be referred to as DC offset distortion at subcarrier 0. The DC offset is estimated based on the two local oscillator (LO) offset-corrected long training symbols that are components of an OFDM preamble. Since the OFDM spectrum may contain severe carrier leakage, the DC estimation based on subcarrier 0 of one OFDM symbol may be distorted and not useful. Thus, the difference of two successive DC estimations is used, canceling out the constant carrier leakage value at subcarrier 0, but retaining some value representing the receiver DC offset (signal r(n) in FIG. 3).
As shown in FIG. 3, the sum of all 34 input time domain samples is computed for the long training symbols x(n) (from the long training symbol buffer 305) by the summation unit 320 to generate a signal, s(n). The signal s(n) is fed through a delay unit 325 and subtracted (block 326) from the current value of s(n) to generate signal r(n).
To calculate the true DC level at the input (i.e., the output of the analog-to-digital converters), the output r(n) is processed to compensate for the Local Oscillator offset compensation (the difference between the transmitter and receiver local oscillators). To accomplish this, the LO offset phasor 328 is down sampled by a factor of 64 (block 330) and complex conjugated, the difference of two consecutive phasor values is calculated (block 340), and the result q(n) is multiplied (block 327) with r(n) to produce t(n). The LO offset frequency 343 is then processed with a compensation factor 345 and multiplied (block 328) with t(n). The result is the receiver DC offset estimate DCRX 329. It should be noted that block 340 comprises a delay unit 332 and subtraction unit 333 that operate in a manner similar to that of delay unit 325 and subtraction unit 326.
The remainder of the circuitry in FIG. 3 is designed to compensate for the DC offset DCRX 329, local oscillator offset (LOO), and the effects of subcarrier rotation. The computed receiver DC offset estimate DCRX 329 is passed through the compensation processing block 380 to calculate the DC contribution in the frequency domain (Davg(i)). Block 350 determines the DC power contribution to each subcarrier (sinc shape) as a function of the LO offset frequency 343. Block 360 is a compensation factor to compensate for the effect of the sub carrier rotation. The down sampled local oscillator offset phasor is averaged over two samples (block 358) to produce the local oscillator offset used by block 380 (block 380 comprises multiplier units 365-367).
The buffer long training symbols 305 are also delayed (block 370) and summed (block 371) to derive the required 64 time samples for input to the FFT block 372. The DC contribution in the frequency domain (Davg(i)) is then added (block 385) to the output of the fast fourier transform (performed by FFT 372) to generate the DC free channel estimation 390. It is noted that the DC free channel estimation 390 is obtained after removing the training data symbols per subcarrier. The removal of the DC offset from each subcarrier should take into account the fact that the Long Training Symbols are averaged over two DC estimations (done by means of block 358).
In FIG. 3, the DC offset estimation method is based on the carrier frequency offset, and at least two repeated training symbols or OFDM symbols are required for DC offset estimation. However, the preambles and the OFDM symbols in 802.16e standard, for example, do not have the ideal repetition feature, thus, the conventional DC offset estimation method based on the carrier frequency offset cannot be applied in 802.16e standard.